The term "sliver" is here used to refer to the fiber band or strip which generally may be drawn from a "can", is drafted in a drafting frame and is then subjected to a twisting operation in, for example, a ring-spinning station. The drafting frame can be provided all along a ring-spinning frame and, at each ring-spinning station, the sliver is fed to the spinning mechanism through a plurality of pairs of rollers of the drafting frame, emerging at the output pair of rollers.
The fiber band or strip is referred to here as sliver, and can also be referred to as roving, the difference between a roving and a sliver generally being the fact that the roving has a slight twist previously imparted to the fiber band or strip before it enters the drafting frame. The present invention is applicable to roving as well and the term "sliver" as here used is intended to apply to fiber bands or strips which are collections of individual fibers of limited length.
Such fiber bands may be subjected to a bundling between the drafting frame and the twist-imparting mechanism. The purpose of such bundling is to make the strand consisting of a multiplicity of fibers, more compact at the inception of twisting. Fibers which project out of the strand or in directions transverse or at an angle to the main direction of the strand are usually drawn back into the body of the strand by such compaction.
In the so-called double-belt drafting frame of DE 43 23 472 C2, a bundling of the strand is described in which the sliver leaving the output roller pair of the drafting frame is subjected to a suction stream which is applied transversely to the travel direction of the strand over a length of a fiber bundling zone. The strand passes over a row of perforations subjected to suction for this purpose. The size of the perforations determines the width of the bundles and compacted roving.
To reduce the wear of the pressure roller in this latter drafting frame, it is customary to induce an offset of the roving from side to side as it is introduced into the drafting frame. This back and forth motion of the roving is referred to here as a traversing motion, and causes the strand to move back and forth during its passage through the drafting frame transversely to the travel direction. When a suction compaction of the prior art type is applied, this traversing motion must be excluded or means must be provided to enable the row of suction bores to remain effective in spite of the traversing motion. In practice, this can be achieved by guidance of the strand with a funnel-shaped guide or by limiting the traversing motion or by conceding a lack of ability to effectively bundle the strand.
In the double-belt drafting frame of DE 197 22 528, the fiber-bundling zone is provided downstream of the output roller pair of the main drafting region. In this case, a transport means, for example a belt, is provided with the suction perforations which are enlarged in a direction transverse to the transport direction. Thereafter, the strand is subjected to suction from a series of finer perforations.
While this system represents an improvement over earlier arrangements, it is also a compromise because the sizes of the larger and smaller perforations are nevertheless limited with respect to the potential traversing motion and the possible stroke of the traversing action and this system as well limits the traversing width or stroke in the sense previously described.
A spinning machine has also been described (see DE 196 23 824 A1) in which a convergence is generated in the suction zone between two strands. A disadvantage in this system is that it is not possible to exclude an effect of suction from one row of perforations on the other strand which might tend to defeat the compaction effect.